While Spawn is the greatest area of my collecting expertise, simply because I dedicated so many resources to compiling a complete, graded run, I will admit that I am not very knowledgeable at all about original art.
Honestly, the perfect person to ask would be @Doc Joe, but I don't think he's around much anymore. He actually sold most of his Spawn collection to me
Tbh I tagged newshane, but would think that this would warrant its own thread in the OA forum or that they'd be cool with it.
That's the first place I'd check, but idk, I don't really comment enough there but sounds like the only place for current answers.